Atmospheric noise at VLF/LF is highly impulsive and nonstationary over any time interval greater than 10 to 15 minutes. Because of the excellent propagation of energy in this frequency range, the noise, which is produced by lightning discharges, is characterized by high dynamic range impulses on the order of 60 dB or more over that of noise components which are Gaussian distributed. The noise in general can be described as a low amplitude Gaussian type background noise in combination with that of the impulsive noise. Because of the nonstationary characteristic of VLF/LF atmospheric noise, attempts have been made in the past to simulate its characteristics in such a manner that communication systems could be elevated with repeatability.
In the past, attempts to produce given amplitude probability distributions used a single pseudo random sequence generator (PRSG) feeding several NAND gates to produce pulses of fixed probability. These pulses of fixed probability were linearly summed in a "Summing Bus" consisting of resistors of different value and an operational amplifier. Each pulse of fixed probability had assigned a specific resistor. Gaussian noise was added to the noise at the "Summing Bus" in cases where dynamic range was not required. The Gaussian noise was used to amplitude modulate the pulses for added randomness. This technique suffered from the disadvantages that the single PRSG did not allow the production of a pulse with significant temporal statistical independence. The probability of the pulses produced were fixed. The amplitudes of the pulse could be changed only by replacing one of the resistors in the "Summing Bus". Due to the lack of statistical independence of pulses (i.e., two pulses had a joint probability) there was great difficulty in attempting to fit amplitude probability distributions commonly seen in VLF/LF spectrum. Also the use of a "Summing Bus" and amplifier produces an unwanted coherent noise which limits the dynamic range. The action of the single PRSG and NAND gates produce a transient noise on the "one shot" pulse lines. Since this noise on each of the lines is the same, it was coherently added in the "Summing Bus".